A new study published in 2025, titled “Unwelcome neighbours: Tracking the transmission of Streptococcus equi in the United Kingdom horse population”, offers critical insights into how strangles, one of the most contagious equine diseases, is transmitted across the UK.
Shifting Carriers: Not What We Thought
Until now, long-term, sub-clinical carriers—horses silently carrying the disease well after infection—were presumed key to strangles’ persistence. However, this study overturns that belief: recently infected or recently recovered horses are, in fact, the main drivers of ongoing spread
Genomic Sleuthing Unlocks the Secrets
Researchers analysed the genomes of over 500 S. equi isolates collected between 2015/16 and 2022. Through advanced methods—including phylogenetic mapping, clustering (BAPS), and transmission tracing (iGRAPH, Transphylo)—they identified two dominant bacterial lineages and mapped how these evolved and spread over time
Crossing Borders: Across the UK
Strikingly, nearly two-thirds of transmission events occurred between horses in different UK regions. One protracted 6-month transmission chain linked animals across England, Scotland, Wales, and Northern Ireland, underscoring how horse movement fuels nationwide spread
Symptoms Aren’t Enough
The study highlights a pressing issue: merely observing a horse’s recovery isn’t enough to declare it clear of infection. Many cases involved horses that appeared symptom-free but were still shedding the bacteria. The authors strongly recommend laboratory-confirmed clearance, particularly via guttural pouch testing, to ensure the disease isn’t silently passed on
A Call to Action for Equine Health
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Mandatory post-outbreak testing rather than reliance on symptom resolution.
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Stronger biosecurity and regulated movement, especially across regions.
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Genomic surveillance as a real-time tool for tracing outbreaks and guiding interventions.
Collaborative Expertise
This impactful research involved a multidisciplinary team from the Royal Veterinary College, University of Cambridge, the Swedish University of Agricultural Sciences, and Redwings Horse Sanctuary—showcasing how collaborative science can directly benefit equine welfare